1. Field of the Invention
The present invention relates to an image processing apparatus, more particularly, to an image processing apparatus for generating one wide dynamic range image from a plurality of images photographed under different exposure conditions.
2. Related Art Statement
Conventionally, there have been proposed various image processing apparatuses for synthesizing a plurality of images photographed under different exposure conditions, thereby generating an image having a wide dynamic range. However, when an image having wide dynamic range synthesized by the image processing apparatus is displayed on a monitor or a printer whose density range is relatively narrow, the dynamic range is compressed.
Generally, the compression of the dynamic range is executed in accordance with characteristics pursuant to a normal γ-characteristic (similar to a logarithmic characteristic) so as to prevent, as much as possible, the contrast in a main object to be photographed and the background from being degraded.
Japanese Unexamined Patent Publication No. 5-314253 disclosed as an example of the above-described technique, in which the compression for high luminance areas is executed in accordance with a characteristic proportional to the logarithm of luminance, and the compression for low luminance areas is executed in accordance with a characteristic almost proportional to the luminance.
An example of known dynamic range compression is illustrated in FIG. 16.
In this example, that a wide dynamic range image is formed on the basis of a short-time exposure signal SIGSE and a long-time exposure signal SIGLE for which the exposure ratio is 1 to 8.
In this instance, if a denotes the incident light level at which the long-time exposure signal SIGLE reaches saturation (100%), it is possible to obtain an incident light level 8α at which the short-time exposure signal SIGSE reaches the saturation.
First, the output level of the short-time exposure signal SIGSE is increased by eight times and then the short-time exposure signal SIGSE is combined with the foregoing long-time exposed signal SIGLE to form a wide dynamic range signal SIGWDR.
Next, to display the wide dynamic range signal SIGWDR on a monitor, a printer, or the like whose density range is narrow, the output level is compressed proportionally to the incident light level in the low output level portion of SIGWDR and logarithmically to the incident light level in high output level portion of SIGWDR, thereby finally obtaining a corrected signal SIGLST.
With the compression characteristics described above, the contrast of the wide dynamic range image is nearly preserved in the low luminance areas, but is reduced in the higher luminance area because the saturation effect causes smaller changes of the output level as the luminance continues to increase.
In a typical photograph, a main object of interest is not necessarily in the low luminance area. For example, if a person is photographed against a dark scene using a strobe, the area around the person will exhibit a relatively high luminance. With the conventional compression technique, the contrast of the person will be degraded and a flat or dull image will result.
According to the foregoing conventional technique, the same compression characteristic is used irrespective of image features such as edge and luminance distribution. The result may be an image having wide dynamic range but low image quality.